I think that is the main takeaway as to why roleplaying as the Warrior of Light isn't recommended: Playing with others as your character.
https://ffxiv-roleplayers.com/topic/21696-rping-wol-fc-question/?tab=comments#comment-310672
This particular post From Hydaelyn Roleplayers I feel sums up the reservations pretty well: "Anyone claiming that their character is a Warrior of Light is basically claiming that their character is one of the most important people to exist in Hydaelyn."
The WoL is literally superhuman, and that on its own is polarizing. The amount of achievements, connections, abilities, and so on are extremely beyond the typical person. It also leads to issues as to what would challenge them, the potential consequences of crossing them, and other lore related things related to a character that, for one reason or another, decides to stand up to the Warrior of Light or not do what they say, which only grows should the Warrior of Light get thwarted. Characters based on the Warrior of Light, because they are working off of someone already far overpowered, tend to be overpowered themselves, and have the very plot of the game as something to lord people over with, which to many RP communities, is an unfair advantage.
That said, making someone overpowered or godmodding their actions is stuff that would affect how well a character is written even if they weren't the Warrior of Light.
